Reactivity of the nitro-group of a pi-conjugated polymer upon the interface formation with chromium: a photoelectron spectroscopy investigation

X-ray photoelectron spectroscopy was used in order to investigate the interface formation between chromium and thin films of a newly synthesised pi-conjugated polymer P(PA-pNO(2)PA); the investigated material is a block copolymer consisting of phenylacetylene (PA) and para-nitro phenylacetylene (pNO(2)PA) units {[-(CH=C-R)(n)-(CH=C-R')(m)-](N); R = C6H5, R' = C6H4NO2}. Chromium was evaporated stepwise in situ onto the polymer surface and the XPS spectra of the substrate polymer and of the metal overlayer were studied as a function of increasing chromium thickness. In the early stages of the interface formation, chromium was found to interact mainly with the nitrogen atoms of the -NO2 groups, that are reduced to nitride-like species; the N-O bond is broken, as evidenced by the modifications occurring in the N1s and O1s spectra. The changes in the C1s spectra upon increasing metal deposition suggest the formation of a chromium-arene pi-complex at low chromium coverage and of carbide-like species at higher chromium thickness.